Referring to FIG. 1, it illustrates the basic circuit of a conventional bust-boost voltage regulator. In reference to FIG. 2, when the switch transistor SW1 is in a conducting state (on-state), the energy provided by the input power supply Vin is stored into the inductor L, and, at the same time, the diode D1 is off. Now turning to FIG. 3, when the switch transistor SW1 is switched off, the energy stored in the inductor L produces an electromotive force in a direction reverse to the input power supply Vin to charge the capacitor C, thereby forcing the diode D1 in a conducting state. Accordingly, the direct current (DC) input power supply Vin is temporarily converted to an alternating current (AC) power and dropped to output a voltage Vo, further providing the power required by a load RL.
When the diode D1 in the above-identified rectifier circuit is conducting, the diode D1 generates a barrier voltage which consumes and wastes some power on the diode D1 itself. The power waste is particularly noticeable at a low output voltage, for example at the output voltage Vo of 1.5V. Under this condition, even with the use of Schottky diodes, it will still generate a barrier voltage of 0.4V±0.1V, resulting in a poor conversion efficiency.
In order to solve the drawback of such a non-synchronous rectifier circuit, the US patent U.S. Pat. No. 8,526,202, for example, provides a synchronous rectifier circuit in which the diode D1 is replaced by another switch transistor. The switch transistor can be a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and so forth. It is well-known that, when a field-effect transistor is conducting, it produces a linear region (a linear voltage-current relationship), and the on-resistance is about 50 mΩ. Assume that the conducting current is 2 A, the resulting voltage drop is merely 0.1V. It thereby improves the disadvantages of the conversion loss due to a high barrier voltage of the diode D1 in the conventional circuit. Such a circuit, in which the gate voltage of the switch transistor needs to be maintained in a phase synchronization with the rectified voltage, is thus named “synchronous rectifier circuit”. The two switch transistors in the mentioned synchronous rectifier circuit must take turns to be conducted or cut off alternatively at each time point. If the two switch transistors are both in a conducting state at the same time, it may cause the damage of the switching power supply.
The above-identified patent utilizes the voltage difference at the two ends of the switch transistor to control conducting or cut-off states. It is equipped with a comparator with an offset voltage at one end to constitute a structure similar to a zero current detector circuit. Through this approach, the purpose of synchronous rectification can be obtained. However, when the circuit is continuously operated in a conduction mode, the switch transistor at the secondary side will be burned and damaged. Therefore, how to accurately control the switching timing and to enable the circuit to operate in a continuous conduction mode, in fact, is the design subject to achieve for a synchronous rectifier circuit.